Starches have been used for many years in the food industry to provide processed foods with improved product appearance and increased consumer appeal. Starches can impart desired properties to food products ranging from shape retention and sliceability in puddings made with pregelatinized starch to good grain and pulpy texture in sauces and gravies made with pregelatinized texturized starches.
The diversity in processed foods has created the need for starches exhibiting specific properties in specialized applications. The sources of starch include: roots (e.g., potato), root-like or tuber plants (e.g., tapioca), and cereals (e.g., corn or wheat). These starches when used in processed foods may be categorized as unmodified (generally in granular state), modified (chemically-modified starch generally retaining granular integrity), pregelatinized modified starches, or pregelatinized texturized starches. Unmodified starch is rarely used for its ability to swell and form a viscous paste (i.e., to gelatinize) because unmodified starch is known to be unstable with respect to granule breakdown (degradation or dissolution). Unmodified starches thus have desirable effects on product quality including gummy or slimy texture when gelatinized in water; unstable paste viscosity on cooking or when subjected to shearing action of pumps, agitators, colloid mills, etc; and gelling of an overcooked paste upon cooling.
Chemical modification is used to improve the properties of unmodified granular starch. The modified starches remain in the granular form (i.e., they are not discrete bulky particles as in pregelatinized starches). Modified starches must possess sufficient consistency so that they will resist degradation of the granules by heat and/or shear.
A particular type of modification that has found great utility in the food industry and the other industries has been designated cross-linked starch. This chemically modified starch type is conceptualized as the toughening of the starch granule. The benefit of these cross-linked starches is that when they are heated in water they swell but the swollen granules remain intact.
A major drawback associated with chemical crosslinking is the progressively decreasing swelling power of the starch granules with increasing crosslinking; the ultimate level of crosslinking being characterized by creation of a starch which is a resistant to gelatinization; i.e., that has no swelling power.
Preparation of pregelatinized starches involves chemical modification of starch granules, subsequent gelatinization in hot water usually above the particular starch's gelatinization temperature (the preferred gelatinization and drying process is drum drying), swelling, bursting of the granules, and drying and milling into a fine-mesh product. The resulting product easily swells in water forming a relatively smooth paste similar in texture to pie filling (i.e., not whole, intact starch granules). On the other hand, a coarse mesh product creates another category of texturized starches, i.e., starches that find utility where the desired food product consistency is pulpy or grainy as in spaghetti sauce or apple sauce (i.e., the particles are not whole, intact granules).
Chemical modifications to produce texturized starches have, in general, the major drawback of difficulty in controlling the degree of crosslinking. Undercrosslinking results in products that are not stable to heat, acid, or shear; are lacking sufficient texture and binding power; and exhibit high swelling power and associated gummy or slimy texture. Conversely, overcrosslinking results in products with insufficient swelling power which require heat for hydration. Most commercial products are slightly overcrosslinked to provide balance for texture, stability, and swelling power.
All of these starches can be used as precursors to be reacted with xanthan gum under the inventive conditions. When this invention uses the term "starch", all food starches in all forms are meant.
Xanthan gum is an exocellular polysaccharide produced by the microorganism Xanthomonas campestris by well known processes. KELTROL F.RTM. is commercially available food grade xanthan gum from Kelco, San Diego, California.
Patent application Ser. No. 785,614 filed April 7, 1977 (U.S. Dept. of Commerce, NTIS Publication PB-272,384 describes a xanthan gum and starch composition used as a low-temperature thickener for jellies, puddings, and similar foodstuffs. This process is highly dependent on xanthan gum's ability to allow free starch constituents to diffuse from unmodified granular starch in an aqueous medium at pH 3.0. to 10.0 and at 25.degree. to 90.degree. C. The resulting mixture when dried to a powder is taught to be readily soluble in both hot and cold water and stable at pH 3.0 to 10.0. The viscosity of the xanthan gum/unmodified starch mixture is shown to be unaffected by the drying process; thus this step was not viewed as integral to free-starch diffusion from the granules. The end product of this process was suggested for use in foodstuffs where improved gelation properties are desired and a smooth jelly-like texture is desired.
It is apparent, then, that the idea of using xanthan gum and starch together in food and other industrial applications is not new. What is one crux of this invention is discovery of a process that enables a novel xanthan gum-modified starch composition to be produced in which the xanthan gum functions as a starch modifier (analogous to a chemical-crosslinking agent) but whose functionality is not restricted by the type of presursor starch used. The end products are uniquely characterized by increased resistance to dissolution in water, by increased acid stability, and by heat and shear stability. These characteristics have previously generally been associated only with pregelatinized modified starches or with pregelatinized texturized starches.